1. Field of the Invention
The present invention relates to a fuel injection control device of an engine.
2. Description of the Related Art
In a fuel injection type engine, the basic amount of fuel injected by a fuel injector is usually calculated from the engine speed and the level of vacuum in the intake passage, or from the engine speed and the amount of air fed into the engine cylinder, and the actual amount of fuel injected by the fuel injector is feedback-controlled so that the air-fuel ratio of mixture fed into the engine cylinder becomes equal to a predetermined desired air-fuel ratio, for example, the stoichiometric air-fuel ratio, by correcting the basic amount of fuel on the basis of the output signal of the oxygen concentration detector (hereinafter referred to as an O.sub.2 sensor) arranged in the exhaust passage of the engine. Nevertheless, even if such a feedback control is carried out, when the amount of fuel injected by the fuel injector is abruptly increased as at the time of acceleration, the amount of fuel adhering to the inner wall of the intake port in the form of a liquid fuel is increased, and since this liquid fuel is not fed into the engine cylinder immediately after adhering to the inner wall of the intake port, the air-fuel mixture fed into the engine cylinder temporarily becomes lean. Conversely, when the engine is decelerated, the absolute pressure in the intake port becomes low, and as a result, since the amount of vaporization of the liquid fuel adhering to the inner wall of the intake port is increased, the air-fuel mixture fed into the engine cylinder temporarily becomes rich.
Consequently, in a fuel injection type engine, the amount of fuel injected by the fuel injector is usually increased at the time of an acceleration and decreased at the time of a deceleration, so that the air-fuel ratio of the mixture fed into the engine cylinder becomes equal to a desired air-fuel ratio, for example, the stoichiometric air-fuel ratio, even if the engine is operating in a transition state such as an acceleration state and a deceleration state. Consequently, in such a fuel injection type engine, the air-fuel ratio of mixture fed into the engine cylinder is controlled so that it becomes approximately equal to the desired air-fuel ratio, regardless of the operating state of the engine.
Nevertheless, in such a fuel injection type engine, blowby gas and lubricating oil, for example, pass through the clearance between the valve stem and the stem guide of the intake valve and flow into the intake port, and thus, when the engine is run for a long time, carbon particles, etc., contained in the blowby gas and the lubricating oil are gradually deposited on the inner wall of the intake port and the rear face of the valve head of the intake valve. These deposited carbon particles, i.e., the carbon deposit, have a physical characteristic of retaining liquid fuel, and thus, if the carbon deposit is deposited on the inner wall of the intake port etc., the amount of liquid fuel adhering to the inner wall of the intake port, etc., is increased, and this increases the time taken by the liquid fuel to flow into the engine cylinder, after the liquid fuel adheres to the inner wall of the intake port, etc. Consequently, although the air-fuel ratio of mixture fed into the engine cylinder can be controlled so that it becomes approximately equal to the stoichiometric air-fuel ratio, regardless of the engine operating state, while the engine is relatively new, if the deposit is deposited on the inner wall of the intake port, etc., after the engine has been run for a long time, since the time taken by the liquid fuel to flow into the engine cylinder is increased, as mentioned above, the air-fuel mixture fed into the engine cylinder becomes lean at the time of acceleration. In addition, since the amount of the liquid fuel adhering to the inner wall of the intake port etc. is increased, the air-fuel mixture fed into the engine cylinder becomes rich at the time of deceleration. At this time, since the amount of the deposit is increased, the air-fuel mixture becomes even leaner at the time of acceleration and even richer at the time of deceleration. In this case, for example, the leaner the air-fuel mixture at the time of acceleration, the longer the time for which the air-fuel mixture remains lean.
Consequently, in a known fuel injection type engine, the time during which the air-fuel mixture becomes lean (hereinafter referred to as a lean time) within a fixed time after the accelerating operation of the engine is started and the time during which the air-fuel mixture becomes rich (hereinafter referred to as a rich time) within the fixed time after the accelerating operation is started, are calculated, and the acceleration increase in the amount of fuel fed by the fuel injector is corrected on the basis of the lean time and the rich time, so that the air-fuel ratio of mixture fed into the engine cylinder becomes a desired air-fuel ratio even if the accelerating operation of the engine is carried out (see U.S. Pat. No. 4499882).
As mentioned above, at the time of acceleration, the lean time becomes longer as the air-fuel mixture becomes leaner. Consequently, at this time, when comparing the lean time with the rich time, if the lean time becomes longer than the rich time, it appears that the air-fuel mixture becomes lean at the time of acceleration. In practice, however, in some cases, when the lean time becomes long, the rich time also becomes long. In such cases, when comparing the lean time with the rich time in the fixed time after the accelerating operation of the engine is started, as in the abovementioned known fuel injection type engine, the lean time becomes equal to the rich time, and thus a problem occurs in that a wrong determination is made, i.e., that the air-fuel mixture has not become lean.
In addition, where the air-fuel ratio of mixture is maintained at approximately the stoichiometric air-fuel ratio at the time of acceleration, the air-fuel mixture alternately becomes lean and rich at an approximately fixed time frequency, even during the accelerating operation of the engine. In this case, when comparing the lean time with the rich time within the fixed time after the accelerating operation of the engine is started, as in the above-mentioned known fuel injection type engine, the lean time becomes longer than the rich time or the rich time becomes longer than the lean time in accordance with the amount of time predetermined as the above-mentioned fixed time. As a result, another problem occurs in that a wrong determination is made, i.e., the air-fuel mixture has becomes lean or rich, although this is not the actual case.